A “boiler bank” is a component in the circulation system of various types of utility and industrial steam generators that acts as a heat sink to reduce flue gas temperature. The boiler bank includes a steam drum and one or more lower drums interconnected by a plurality of tubes. Each of the drums of a boiler bank may be generally cylindrical and is provided with a plurality of radially extending bores arrayed in rows and columns on the cylindrical face thereof. Ends of the tubes are secured within the radially extending bores to provide fluid communication between the steam drum and the one or more lower drums.
A typical steam drum has a diameter of between three and six feet, and a typical lower drum has a smaller diameter. Also typically, the spaced tubes may be about twenty feet long, though other lengths are certainly known. Typical tubes often have a diameter of about two to three inches and a varying wall thickness depending on pressure and operating temperatures.
Original installation of a boiler bank tube typically involves inserting ends of the tube in the radially extending bores in each of the drums. In many cases, the ends of the tubes are swaged down to a required outside diameter to fit the bore. Once inserted, the ends of the tubes may be subjected to a rolling process. The tube rolling process utilizes a mandrel which is inserted into the end of the tube from the inside of the drum into which that end is inserted. This tool exerts an outward radial force on the inside surface of the tube causing the tube to expand and form a gas tight seal with the drum. During this process, the wall thickness of the tube in the area of the seal is slightly reduced and the tube end may be flared inside the drum or recessed in a counterbore.
Those skilled in the art will understand that the bores in the drum are cylindrical and are not chamfered. The cylindrical wall of the drum, commonly referred to as the drum sheet, usually has a thickness of between 2.5 to 5.0 inches.
Periodically, the individual tubes of the boiler bank require replacement due to long term exposure to high gas temperatures and velocities. Both erosion and corrosion play a role in reducing the life of the boiler bank tubes. When the wall thickness of a tube is reduced below the minimum wall thickness allowed by ASME code requirements, the tube is generally removed and replaced. The removal process typically involves cutting off the tube to be replaced at an axial point outside of the drum, thus leaving a relatively small portion of the tube (a “tube stub”) attached to the drum. Because the tube is flared on the inside of the drum the removal of the remaining tube stub is generally performed from the inside of the drum.
The difficulty involved in removing the tube stubs will be better understood by consideration of the size of the drum, the tenacity of the engagement between the tubes and the drum sheet, the necessity of performing the work inside the drum because the flared portion of the tube is inside the drum, the size of the workman performing the work, and the location of obstructions within the drum. The obstructions include centrifugal separators, screen dryers and various pipes.
The removal of boiler bank tubes results in a significant number of man hours each year. The removal of boiler bank tubes is a tedious process and, in many cases, results in damage to the drum sheet. The repair or replacement of the drum sheet can be very expensive.
Several methods have been employed for removing boiler bank tubes with varying success. The most common prior art method involves the use of a brazing tip on an oxy-acetylene torch to cut a narrow slot along the length of the tube stub. Once the slot has been formed, the tube stub can generally be hammered or chiseled to free the tube stub from the drum sheet, which may score the drum sheet and prevent the new, replacement tube from ever obtaining an adequate seal with the drum sheet. In addition, this is a time consuming process and requires a skilled boilermaker in order to achieve the desired success.
Other prior art approaches have included various hydraulic tools to remove the tube stub. Some of them may be adequate for tubes having especially thin walls or which engage especially thin surfaces, such as found in some particular types of heat exchangers. One known approach is to insert a gripper into the tube end and force the gripper outward to embed a series of screw type threads into the inside surface of the tube. As the threads grasp the tube, the tool simultaneously attempts to pull the tube and break the seal between the tube and the drum sheet. Although this tool has worked for smaller diameter tubes, it is not wholly satisfactory for removal of boiler bank tube stubs. The large tubes used in such applications and the larger area of contact between the tube and the drum sheet make the removal task more difficult. At least partly because of the larger area of contact, the threads of the tool will loose their grip on the tube before the tube separates from the drum sheet. An additional factor is that such a prior art tool functions by applying an outward force to the inside of the tube. This is inherently counterproductive because the outward force pushes the tube wall firmly against the drum sheet from which separation is desired.
Thus, there remains a need for an apparatus that will facilitate the quick removal of tube stubs from a drum with minimum risk of damage to the drum, and that requires less skill to use than conventional tube stub removal techniques.